Photon counting detector and photon counting method

Abstract

The invention relates to a photon counting detector and a method. The detector (20) comprises a scintillator (21) configured to convert incident gamma radiation into optical photons, a pixelated photodetector (22) configured to detect a flux of optical photons, and a circuit (23). The circuitry (23) is configured to: determine, for each photodetector pixel, a photon count by accumulating the number of optical photons detected by the respective photodetector pixel during an integration time; assigning, for each photodetector pixel, the photon count to an energy bin of a plurality of energy bins by using an energy threshold separating the plurality of energy bins; and for each photodetector pixel or group of photodetector pixels, dynamically adjusting the energy threshold for use in a subsequent iteration based on information regarding an estimated photon count of the photodetector pixel or group of photodetector pixels in the subsequent iteration.

Description

Technical Field

[0001] The present invention relates to a photon counting detector, a corresponding photon counting method and an imaging device comprising a photon counting detector for detecting radiation, in particular gamma radiation and X-ray radiation. Background Art

[0002] Photon counting is typically performed using a photomultiplier tube (PMT) detector, which includes a photosensitive photocathode and a set of "multiplier" anode terminals. The photocathode emits at least one electron in response to the impact of a photon, and the electron(s) then strike successive anode terminals, each such event causing a cascade of electron emissions, thereby creating a multiplication effect that causes a measurable electrical pulse. PMTs are capable of high-speed photon counting. Disadvantages of PMT detectors include: being relatively bulky discrete devices with large optical windows operating at high voltages; and being susceptible to failure due to the vacuum tube design and high operating voltages.

[0003] Silicon photomultiplier (SiPM) devices have been developed to overcome some of these disadvantages and provide photon counting detectors that are easily integrated with silicon-based signal / data processing circuits. In some embodiments, SiPM devices use avalanche photodiodes as light sensors. When biased above its breakdown voltage, the avalanche diode breaks down in response to the collision of a single photon. Such devices are sometimes referred to as single-photon avalanche diode (SPAD) detectors. In a typical SPAD detector, the avalanche photodiode is reverse biased above its breakdown voltage and connected in series with a quenching resistor. The collision of a single photon causes the pn junction to break down in an electron multiplication (i.e., "avalanche") cascade, which flows in the SPAD detector as a measurable current. Since the voltage generated by the current flow on the resistor reduces the reverse bias voltage across the avalanche diode to a level below its breakdown voltage, the current is quenched relatively quickly. Additionally or alternatively, an active quenching subcircuit including (for example) one or more diodes, resistors, and / or transistors can provide faster quenching.

[0004] Digital silicon photomultipliers (dSiPMs) consisting of a matrix of single-photon avalanche detectors (SPADs) are used as photon counting detectors (sometimes also called photon detectors) in high-performance PET / CT systems to detect single gamma quanta. The fast trigger logic of the digital SiPMs is coupled to a time-of-flight-enabled scintillator, which enables temporal resolution in the range of 100ps-200ps. The temporal resolution of these systems is particularly limited by the temporal spread of the scintillation process. New detector concepts using prompt photons have been proposed, which potentially provide significant improvements in temporal resolution below 100ps.

[0005] However, high count rates limit the dynamic range, especially in low attenuation direct beams or in the body periphery. Thus, for high count rates, pulses overlap in time (optical pile-up), significantly degrading the received energy resolution. Summary of the invention

[0006] It is an object of the present invention to solve or at least reduce the problem of detector pixel pile-up.

[0007] In a first aspect of the present invention, a photon counting detector is provided, comprising:

[0008] a scintillator configured to convert incident gamma radiation into optical photons;

[0009] a pixelated photodetector configured to detect a flux of optical photons; and

[0010] A circuit is configured to iteratively perform the following operations:

[0011] - for each photodetector pixel, determining a photon count by accumulating the number of optical photons detected by the corresponding photodetector pixel during the integration time,

[0012] - for each photodetector pixel, assigning the photon count to one of the plurality of energy bins by using an energy threshold separating the plurality of energy bins, and

[0013] - for each photodetector pixel or group of photodetector pixels, dynamically adjusting the energy threshold for use in a subsequent iteration based on information about the estimated photon count of said photodetector pixel or group of photodetector pixels in said subsequent iteration.

[0014] In a further aspect of the invention, a corresponding photon counting method is proposed.

[0015] In yet a further aspect of the invention, an imaging device comprising a photon counting detector as disclosed herein is presented.

[0016] Preferred embodiments of the invention are defined in the dependent claims.

[0017] High count rates limit the dynamic range, especially in low attenuation direct beams or in the body periphery. Because the light yield of very fast scintillators is usually very low, it is not beneficial to use very short integration times (lower than the decay time of the scintillator), which will lead to poor energy resolution and loss of spectral information. Therefore, a compromise should be found to balance spectral separation with count rate capability, which is usually determined by design and cannot be adjusted dynamically for individual scans. In addition, for high count rates, pulses overlap in time and lead to optical pile-up, which deteriorates the received energy resolution.

[0018] The present invention overcomes or at least reduces this drawback by dynamic adaptation of at least the energy threshold to provide the best possible separation of gamma quanta and their individual energies. A pixel-based energy threshold adaptation (signal measured in optical photons) is proposed to obtain an energy histogram with a fixed energy threshold (preferably in keV). This solution also provides a compromise between spectral separation and counting rate capability.

[0019] In one embodiment, the circuit is configured to dynamically adjust the energy threshold and the integration time for each photodetector pixel or group of photodetector pixels for use in the subsequent iterations. This further improves the solution to the trade-off between spectral separation and count rate capability, in particular by combining adaptive and dynamically adjustable integration time (integration length) with automatic energy threshold adaptation to the energy histogram.

[0020] In another embodiment, the circuit is configured to dynamically adjust the energy threshold and / or the integration time for use in the subsequent iteration based on one or more of the following for each photodetector pixel or group of photodetector pixels:

[0021] one or more arrangements of detection devices comprising said photon counting detectors,

[0022] the current of the radiation source of the detection device including the photon counting detector,

[0023] examining the position information of the object relative to the photon counting detector,

[0024] information about one or more earlier photon counts from the same and / or adjacent photodetector pixels and / or from the same and / or adjacent groups of pixels,

[0025] the operating temperature of the photon counting detector, and

[0026] One or more operating parameters of the same and / or adjacent photodetector pixels and / or the same and / or adjacent groups of pixels and / or pixel elements of the same and / or adjacent groups of pixels.

[0027] The operating temperature may be useful since the scintillator light yield and thus the detector sensitivity generally depend on the operating temperature.The operating parameters (eg bias voltage and recharge timing in the case of a SPAD detector) can compensate for the temperature coefficient of the scintillator, for example to generate a stable operating point.

[0028] The circuit may be configured to increase the energy threshold and / or the integration time the lower the estimated photon count is. This dynamic adaptation of the energy threshold and / or integration time to the actual count rate allows for longer integration times and improved energy resolution at lower rates, and shorter integration times at high rates to automatically reduce pile-up effects.

[0029] The circuit may also be configured to adjust the energy threshold and / or the integration time by calculating the energy threshold and / or the integration time based on a function or by obtaining the energy threshold and / or the integration time from a lookup table. This provides a simple and effective implementation.

[0030] In another embodiment, the circuit may be configured to adjust the energy threshold and the integration time by selecting one of several integration modes, each integration mode indicating a corresponding energy threshold and integration time. Thus, for each of the several integration modes, the corresponding energy threshold and integration time may be predetermined, so that only the integration mode needs to be selected (e.g., by a control signal).

[0031] Preferably, the circuit can be configured to: limit the energy threshold and / or the integration time to not exceed the maximum energy threshold and / or the maximum integration time, and not be lower than the minimum energy threshold and / or the minimum integration time, so as to avoid the loss of some photon counts on the one hand, and avoid potential errors from being counted as photons on the other hand.

[0032] The pixelated photodetector may preferably be a silicon photomultiplier (SiPM) detector, wherein each detector pixel comprises an array of silicon avalanche photodiodes (SPADs). Thus, the circuit may be configured to dynamically adjust, for each photodetector pixel or group of photodetector pixels, in addition to the energy threshold, the recharge time and / or bias voltage of the SPAD of the corresponding photodetector pixel or group of photodetector pixels for use in the subsequent iterations. For example, the bias voltage may be increased (e.g., by a few millivolts) to compensate for the negative temperature coefficient of the scintillator. Additionally (or alternatively), the recharge time (SPAD dead time) of each SPAD in the pixel matrix may be reduced (e.g., by a few nanoseconds) to reduce the dead time effect and thereby increase the expected value of the total photon count.

[0033] In one embodiment, the circuit may be configured to apply a dead time correction function to the photon counts assigned to the energy bins, wherein the dead time correction function is dynamically adjusted based on the integration time. For example, a correction factor may be applied as a correction function that increases dynamically with increasing integration time and increasing radiation dose of the incident gamma radiation. This ensures that the ratio of count rates of different energy bins remains substantially constant as the integration time varies.

[0034] In another embodiment, the circuit may be configured to dynamically adjust the energy threshold and / or the integration time (typical value is about 100 microseconds) for each integration period.

[0035] The claimed imaging device comprising a photon counting detector as disclosed herein may be, for example, a PET device, a CT device, a SPECT device, a time-of-flight (TOF) PET device, or a Compton camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] These and other aspects of the invention will become apparent and elucidated with reference to the embodiment(s) described hereinafter. In the accompanying drawings:

[0037] FIG. 1 shows a diagram of a typical decay curve and an integral curve (depending on the integration time) of a conventional photon counting detector.

[0038] Figure 2 A schematic diagram of an embodiment of a photon counting detector according to the present invention is shown.

[0039] 3 shows a graph of the output rate of a photon counting detector and the corresponding correction factor (depending on the integration time) according to an embodiment of the present invention.

[0040] 4 shows a graph of different sets of energy thresholds (depending on integration time) and energy resolutions (depending on gamma-ray energy) for a photon counting detector according to an embodiment of the present invention. DETAILED DESCRIPTION

[0041] A diagram of typical decay curves and integral curves (depending on integration time) of a conventional photon counting detector is shown in Figure 1. Fast scintillators usually exhibit two or more decay components with different weightings (depending on material properties including chemical composition, energy bands, excitation phase, doping and co-doping profiles, etc.). Figure 1A A typical scintillator decay curve 10 is shown, which consists of a short decay component 11 and a long decay component 12 . Figure 1B The integration of this example with different integration times (also referred to as integration lengths) is shown.

[0042] In the case of single gamma counting, the spectral separation strongly depends on the amount of light collected by the pixelated photodetector (e.g., by a SPAD matrix). For example, a short (e.g., 20 ns) integration ( Figure 1B Curve E in FIG. 1 can, for example, collect about 63% of the light, while a longer (eg, 40 ns) integration ( Figure 1B Curve C) in FIG. 4 can for example collect approximately 86% of the light, resulting in an improved energy resolution, with the disadvantage that counting losses are higher at high rates.

[0043] The present invention describes how to overcome this trade-off by automatic energy threshold adaptation to the energy histogram, preferably in combination with adaptive and dynamically adjustable integration time. According to one embodiment, dynamic adaptation of the integration time to the actual count rate is used, which allows longer integration times and improved energy resolution at lower rates, and shorter integration times at high rates to automatically reduce pile-up effects.

[0044] The detection system of CT usually rotates with the source, and the curved detector is usually composed of multiple individual modules, each of which includes multiple sensor tiles. Each sensor tile is electrically, mechanically and thermally connected to a module to provide power, clock, signals, etc. The sensor tile consists of multiple silicon dies, each with multiple pixels, aligned with the scintillation array pixel pitch.

[0045] Figure 2 A schematic diagram of an embodiment of a photon counting detector 20 according to the present invention is shown. The detector 20 generally comprises a scintillator 21 configured to convert incident gamma radiation into optical photons, a pixelated photodetector (represented as a SPAD matrix 22) configured to detect a flux of optical photons, and a circuit 23 (or corresponding unit) configured to further process / evaluate the detected optical photons.

[0046] In more detail, Figure 2 A single pixel of a sensor die is shown, which in this embodiment comprises a SPAD matrix 22 on the one hand and a link 24 to an IO (input / output) module on the other hand. The SPAD matrix 22 collects the scintillation light from the gamma rays. A programmable photon accumulator 25 calculates the sum of each acquisition pulse (in particular a programmable number of samples) and stores it in a histogram cell 26 with a configurable energy threshold. A controller 28 (or processor) can calculate a suitable energy threshold for the histogram stored in, for example, a threshold memory 27, the controller 28 taking into account one or more parameters. The controller 28 adjusts the energy threshold directly and optionally adjusts the recharge logic 29. The resulting histogram data and integration settings can be fed into a downstream data link 24 to the IO module.

[0047] The energy threshold can be implemented in the digital domain using an N-bit comparator, allowing the energy threshold to be adjusted with single detected photon precision. When light is collected by a single optical photon detection microunit, the energy of the detected X-ray quantum depends on the effective light yield of the scintillator and the "integration" time (the number of photon count samples to be accumulated). Assuming a collection efficiency of 1ph / keV (e.g., 1 detected photon / keV; however, depending on the implementation, the collection efficiency can also be higher or lower (e.g., higher or lower by a factor of 2 or 3), the energy threshold can be adjusted in steps of 1keV.

[0048] In one embodiment, the controller may adjust the integration time, threshold, and recharge timing based on feedback from one or more of the following parameters:

[0049] System settings such as acquisition mode (fixed / variable integration)

[0050] System rate estimate derived from tube current, patient information, camera, etc.

[0051] The local rate estimate for the previous integration period (IP) based on the histogram counts of this pixel and / or neighboring pixels

[0052] Operating temperature

[0053] Bias voltage

[0054] Recharge time

[0055] The adaptation goal is to achieve a suitable energy histogram with a constant effective threshold in order to provide a fixed ratio between energy bins for a variable tube current (in the case of a fixed input spectrum).

[0056] It should be noted that the light yield of the scintillator and thus the sensitivity of the detector depends on the operating temperature. Higher temperatures reduce the overvoltage (at a fixed bias voltage) and the light yield of the scintillator also decreases with temperature. These two effects can be compensated together by increasing the bias voltage. Since this process is quite slow (in seconds or tens of seconds), the alternative of changing the threshold and integration time can ultimately be implemented as fast as a few microseconds.

[0057] Increased count rates require shorter integration times and higher thresholds, while increased temperature requires lower thresholds and / or longer integration times to ensure gain stability.

[0058] Spatial and temporal flexibility (essentially down to the single pixel level and IP level) allows adaptive data acquisition with optimal energy-binned spectral separation and minimal dead-time effects.

[0059] A practical example of the present invention describes an implementation that runs at, for example, 200 MHz, while calculating energy estimates at each clock cycle of, for example, 5 ns. In one embodiment, a set of variable integration times (e.g., 20 ns, 30 ns, 40 ns, and 50 ns) can be defined, resulting in a set of four different energy thresholds for each energy bin. In this example, three energy bins of 30 keV, 60 keV, and 90 keV are used. All numbers are arbitrarily chosen and can vary in practical implementations.

[0060] Therefore, the histograms of the different integration modes require a suitable dead time correction function that is dose dependent (which can be implemented by a two-dimensional function or a look-up table).

[0061] FIG. 3 shows the output rate of a photon counting detector according to an embodiment of the present invention ( Figure 3A ) and the corresponding correction factor ( Figure 3B ) (depending on the integration time). It can be seen from FIG3 that the output rate (i.e., sensitivity) depends on the effective integration time, which also results in different correction factors for each energy bin (only one energy bin is shown here) and integration setting. This dependence of sensitivity on the integration time, the intrinsic decay curve and the gamma-ray spectrum gives rise to the following: Figure 3A The dead time model shown and Figure 4B The resulting correction factors (depending on the integration time) are shown.

[0062] The advantage of increasing the integration time at a lower rate can be seen in Figure 4, as longer integration results in better energy resolution and improved spectral information for each energy bin. Figure 4 shows a graph of different sets of energy thresholds (depending on integration time) and energy resolution (depending on gamma-ray energy) for a photon counting detector according to an embodiment of the present invention. Figure 4A In particular, three sets of adaptive energy thresholds are shown depending on the desired physical threshold (in keV) for a different set of integration times. Figure 4B Different energy resolutions depending on the gamma-ray energy are shown.

[0063] One of the features of the present invention is the dynamic adaptation of the energy threshold at the level of a single optical photon depending on the chosen integration setting. The integration setting can be changed dynamically, for example, after each integration period (IP), and for each pixel individually. In general, the entire configuration of the sensor can be written to the background during one IP, and the entire configuration of the sensor can be automatically activated in the next IP. In one embodiment, this can also include, among other things, a recharge time and / or an integration time and / or a SPAD suppression map. With this suppression map, each SPAD of the pixel matrix can be enabled or disabled via a sequential access memory (SAM). It is typically used to disable high-noise SPADs during initialization, but can also be used to dynamically fine-tune the gain between integration periods (which may be in the range of 50μm to 500μs, typically about 100μs).

[0064] For additional fine tuning of the energy threshold (operating at single optical photon resolution), additional parameters that affect the light collection efficiency can be adjusted, in particular for sub-photon threshold adaptation: Changes in the recharging logic with different recharging timings affect the effective dead time of each SPAD and thus the acquired gain. Moreover, small changes in the rejection map can also similarly adjust the effective gain. In contrast to the two mentioned parameters that can be used at the single pixel level and the single IP level, the bias voltage for each sensor die or sensor patch can be adjusted for fine proportional gain adaptation (affecting a larger sensitive area).

[0065] For downstream data correction, the integration settings used for each acquired histogram storage and transmission can be provided (e.g., reflected in the table below). This can be done at the pixel level and the IP level for maximum granularity and flexibility. For simplicity, these settings can also be set commonly for multiple pixels of a die, such as row-based selection, column-based selection, sub-pixel-based selection, or die-based selection, which can reduce the payload of the transmitted data.

[0066] In one embodiment, four (or any other number) integration modes may be used, which requires only two extra bits in the data stream, which is still a factor of 20-40 smaller than the histogram information (the energy bin counter consumes 40-80 bits). Assume that each integration mode identifies a set of predefined energy thresholds (and preferably, integration times and / or recharge times), as shown in this exemplary Table 1:

[0067] integral integral Threshold Threshold Threshold [ns] set up For 30keV[ph] For 60keV[ph] For 90keV[ph] 20 0 19 38 58 30 1 23 46 69 40 2 25 50 75 50 3 27 53 80

[0068] Table 1

[0069] The examples of Table 1 are given for three different gamma energies: 30keV, 60keV and 90keV. The unit [ph] corresponds to the number of photons detected in the gamma pulse. The scaling factor for keV is known, but is different for each setting. As mentioned above, the integration time can also be pre-set by the system via the IO module, so that the local controller implements the given setting (rather than autonomous adjustment).

[0070] Table 2 below shows an example of the expected photon energy for a 30 ns integration obtained by rejection map adaptation when a total of 128 SPADs are used for the matrix:

[0071] Threshold Threshold Threshold Enabled Units For 30keV[ph] For 60keV[ph] For 90keV[ph] 93％ 23.0 46.0 69.0 94％ 23.2 46.4 69.6 95％ 23.4 46.8 70.2 96％ 23.6 47.2 70.8

[0072] Table 2

[0073] Table 3 below shows an example of the expected photon energy for a 30ns integration obtained by recharge time adaptation. Shorter recharge times reduce the effective dead time of the SPAD matrix and increase the gain. For high photon energies, this effect is stronger due to the high optical photon flux at the start of the flash:

[0074] SPAD recharge time Threshold Threshold Threshold [ns] For 30keV[ph] For 60keV[ph] For 90keV[ph] 4 23.3 46.9 70.5 5 23.2 46.6 70.0 6 23.1 46.3 69.5 7 23.0 46.0 69.0

[0075] Table 3

[0076] Bias voltage adaptation can additionally be used, in particular to compensate for the effects of temperature drift. For example, a typical temperature coefficient of the SPAD breakdown voltage is in the range of -20 mV / K, which results in a required bias voltage increase of 20 mV (e.g., 26.0 V to 26.2 V). To compensate for the negative temperature coefficient of the scintillator light yield, the bias voltage can be adjusted using overdrive, e.g., 4 mV / K, which would result in a bias voltage of 26.24 V to compensate for the gain loss of 1 Kelvin.

[0077] As an alternative to real-time threshold adaptation in the keV range, in one embodiment it is also possible to maintain an energy threshold (calibrated but fixed, depending on the scan protocol and position) and (depending on the rate) use a set of two-dimensional histogram corrections and integration times within the pre-reconstruction data stream.

[0078] In summary, the present invention proposes dynamic adaptation of energy threshold (preferably, dynamic adaptation of integration time) to provide the best possible separation of gamma quanta and their individual energies. Pixel-based energy threshold adaptation is used to obtain an energy histogram with a stable energy threshold.

[0079] Although the present invention has been described and illustrated in detail in the drawings and the foregoing description, such description and illustration are to be considered illustrative or exemplary rather than restrictive; the present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention by studying the drawings, the disclosure, and the appended claims.

[0080] In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0081] The elements of the disclosed devices, apparatuses and systems can be implemented by corresponding hardware elements (e.g., one or more appropriate circuits) and / or software elements. A circuit is a structural collection of electronic components (including conventional circuit elements, integrating circuits including dedicated integrating circuits, standard integrating circuits, dedicated standard products, and field programmable gate arrays). In addition, the circuit includes a central processing unit, a graphics processing unit, and a microprocessor that are programmed or configured according to software code. The circuit does not include pure software, but the circuit includes the hardware that runs the software as described above. One or more circuits can be implemented by a single device or unit or multiple devices or units or (one or more) chipsets or (one or more) processors.

[0082] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A photon counting detector, comprising: a scintillator (21) configured to convert incident gamma radiation into optical photons; a pixelated photodetector (22) configured to detect a flux of optical photons; and A circuit (23) is configured to iteratively perform the following operations: For each photodetector pixel, a photon count is determined by accumulating the number of optical photons detected by the corresponding photodetector pixel during the integration time, for each photodetector pixel, assigning the photon count to one of the plurality of energy bins by using an energy threshold that separates the plurality of energy bins, and For each photodetector pixel or group of photodetector pixels, the energy threshold is dynamically adjusted based on information about the estimated photon count of the photodetector pixel or group of photodetector pixels in a subsequent iteration for use in the subsequent iteration.

2. The photon counting detector according to claim 1, in, The circuit (23) is configured to dynamically adjust the energy threshold and the integration time for each photodetector pixel or group of photodetector pixels for use in the subsequent iteration.

3. The photon counting detector according to claim 1 or 2, in, The circuit (23) is configured to dynamically adjust the energy threshold and / or the integration time for use in the subsequent iteration based on one or more of the following for each photodetector pixel or group of photodetector pixels: one or more arrangements of detection devices comprising said photon counting detectors, the current of the radiation source of the detection device including the photon counting detector, examining the position information of the object relative to the photon counting detector, information about one or more earlier photon counts from the same and / or adjacent photodetector pixels and / or from the same and / or adjacent groups of pixels, the operating temperature of the photon counting detector, and One or more operating parameters of the same and / or adjacent photodetector pixels and / or the same and / or adjacent groups of pixels and / or pixel elements of the same and / or adjacent groups of pixels.

4. A photon counting detector according to any one of the preceding claims, in, The circuit (23) is configured to increase the energy threshold and / or the integration time the lower the estimated photon count is.

5. A photon counting detector according to any one of the preceding claims, in, The circuit (23) is configured to adjust the energy threshold and / or the integration time by calculating the energy threshold and / or the integration time based on a function or by obtaining the energy threshold and / or the integration time from a lookup table.

6. A photon counting detector according to any one of the preceding claims, in, The circuit (23) is configured to adjust the energy threshold and the integration time by selecting one of several integration modes, each integration mode indicating a corresponding energy threshold and integration time.

7. A photon counting detector according to any one of the preceding claims, in, The circuit (23) is configured to: limit the energy threshold and / or the integration time to not exceed a maximum energy threshold and / or a maximum integration time, and not be lower than a minimum energy threshold and / or a minimum integration time.

8. A photon counting detector according to any one of the preceding claims, in, The pixelated photodetector (22) is a silicon photomultiplier (SiPM) detector, wherein each detector pixel comprises an array of silicon avalanche photodiodes (SPADs).

9. The photon counting detector according to claim 8, in, The circuit (23) is configured to: for each photodetector pixel or group of photodetector pixels, in addition to the energy threshold, dynamically adjust the recharge time and / or bias voltage of the SPAD of the corresponding photodetector pixel or group of photodetector pixels for use in the subsequent iteration.

10. The photon counting detector according to claim 9, in, The circuit (23) is configured to dynamically perform the following operations: the lower the estimated photon count, the more the recharge time is reduced and / or the more the bias voltage is increased.

11. The photon counting detector according to claim 2, in, The circuit (23) is configured to apply a dead time correction function to the photon counts assigned to the energy bins, wherein the dead time correction function is dynamically adjusted based on the integration time.

12. The photon counting detector according to claim 11, in, The circuit (23) is configured to apply a correction factor as a correction function, the correction factor dynamically increasing with increasing integration time and increasing radiation dose of the incident gamma radiation.

13. A photon counting detector according to any one of the preceding claims, in, The circuit (23) is configured to dynamically adjust the energy threshold and / or the integration time for each integration period.

14. An imaging device comprising a photon counting detector (20) according to any one of the preceding claims.

15. A photon counting method, comprising: determining a photon count by accumulating the number of optical photons detected by the corresponding photodetector pixel during an integration time for each photodetector pixel of a pixelated photodetector (22), the pixelated photodetector being configured to detect a flux of optical photons converted by a scintillator (21) according to incident gamma, for each photodetector pixel, assigning the photon count to one of the plurality of energy bins by using an energy threshold that separates the plurality of energy bins, and For each photodetector pixel or group of photodetector pixels, the energy threshold is dynamically adjusted based on information about the estimated photon count of the photodetector pixel or group of photodetector pixels in a subsequent iteration for use in the subsequent iteration.

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